1. Field of the Invention
The present invention relates to an apparatus for driving a variable reluctance motor
2. Description of the Related Art
A variable reluctance motor, which includes a stator having a plurality of salient poles around which exciting coils are wound and a rotor having a plurality of salient poles, is so arranged as to rotate the rotor by means of magnetic attraction forces produced by the salient poles of excited stators, rotor salient poles concerned being attracted by the forces toward these stator salient poles. A rotary torque applied to the rotor acts in the direction to decrease the reluctance between the stator salient poles and the rotor salient poles regardless of the direction of electric currents flowing through the exciting coils. Thus, the acting direction of the rotary torque is determined in dependence on the rotary position of the rotor which indicates the positional relationship between the stator salient poles and the rotor salient poles. For this reason, to run the motor in a desired rotational direction, generally, a PWM signal is applied to either a first pair of switching elements Q1 and Q2 or a second pair of switching elements Q3 and Q4 in an H bridge circuit (FIG. 1), which is interposed between a DC power supply E and the exciting coil L for a respective phase, while an inverted PWM signal is applied to the other switching element pair, so that the first and second pairs of switching elements are turned ON and OFF alternately, thereby successively exciting the stator salient poles of individual phases in a required order for a required rotational angular region of the rotor.
When the H bridge circuit of FIG. 1 is used, an exciting current whose polarity alternates flows through the exciting coil L. In the variable reluctance motor wherein the acting direction of the rotary torque does not depend on the direction of the exciting current, however, it is permitted to cause the exciting current to flow in one direction. From such a viewpoint, motor driving circuits, shown in FIGS. 2 and 3, each using a pair of diodes instead of either one of the switching element pairs in the H bridge circuit shown in FIG. 1, have been proposed in an attempt to simplify the circuit configuration and reduce costs.
According to these proposals, a carrier signal 1' having an amplitude of .epsilon.0 is compared with a current deviation .epsilon. at a central level (OV) of the carrier signal 1', as shown in FIG. 4, to thereby generate a PWM signal of a duty ratio .alpha. represented by equation (1) given below: EQU .alpha.=(.epsilon.+.epsilon.0)/2.epsilon.0 (1)
In the case of applying the PWM signal to, e.g., the switching elements Q1 and Q2 of the driving circuit shown in FIG. 2, when both the switching elements are simultaneously turned on, an electric current i (FIG. 5 (b)), which rises with a time constant that depends on the inductance and resistance of the exciting coil L, flows from a DC power supply E to the switching element Q1, the exciting coil L, the switching element Q2, and back to the DC power supply E. As a result, a positive voltage +E whose magnitude is approximately equal to a DC line voltage E is generated across the coil L. On the other hand, when these switching elements are turned off, the current i, which falls with the time constant of the exciting coil L, flows back to the DC power supply E through a diode D2, the exciting coil L, and a diode D1, thus producing a negative DC line voltage -E across the coil L.
If the ON-OFF duty ratio .alpha. of the PWM signal exceeds a value (e.g. 50%) associated with the time constant of the exciting coil L or the like, the voltage across the coil is alternately changed between +E and -E, as described above, while the PWM signal is turned ON and OFF. In this case, referring to FIG. 4, an average voltage VLa applied to the exciting coil L is given by equation (2). In other words, the average voltage VLa changes linearly with respect to the duty ratio .alpha. and the current deviation .epsilon.. ##EQU1##
On the other hand, if the duty ratio .alpha. of the PWM signal is, e.g., 50% or less, when the current i gradually decreases to a value of "0," after the PWM signal is turned OFF, as shown in FIG. 5, the exciting coil L whose both ends are connected to the switching element Q1 and diode D1 which are in OFF states and to similar circuit elements Q2 and D2 is brought in a floated state, so that a voltage VL across the exciting coil L is rendered unstable. Therefore, if the duty ratio .alpha. is 50% or less, then equation (2) cannot be fulfilled between the average voltage VLa, the line voltage E, and the duty ratio .alpha.. In other words, the average voltage VLa changes nonlinearly with respect to the current deviation .epsilon.. This causes difficulties in controlling the motor.